Beyond Nature: Next-generation ‘Masters of Disguise’

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Chameleon camouflage, a biomimetic technology derived from the reversible shift of skin colors of chameleons through controlling the lattice of guanine nanocrystals, has been extensively studied to improve current camouflage methods originating from pattern painting dating back to the 1940s. Although chameleons have been widely accepted as “masters of disguise”, there are still some other natural color-changers including octopus vulgaris, cuttlefish, and the Andean rainfrog, which show more sophisticated camouflage abilities. They can not only adapt rapidly to the surrounding colors just like chameleons, but also reversibly change their texture or posture to fit the environments, which results in a greater chance of survival. Recently, inspired by animals with dual-stealth capability, emerging progress has been reported to develop smart hybrid materials driven by different stimuli (such as gas, light, magnetism, and so on), to realize the biomimetically dual-responsive technology. Despite these achievements, there still exist some critical shortcomings: (i) the dual-responsive performances of these materials were stimulated by gas, light or magnetism, which are affected by environmental conditions and are weakly controllable; (ii) the color changes were not striking enough to be observed by the naked eye within all visual angles, which lead to moderate detectability.

How to develop next-generation artificial “masters of disguise”? Challenging but Stirring! Our group have focused on this topic 4 years ago. In our previous work, we develop an interactive mechanochromic actuation (color switching) through multilayer surface modification of the soft actuation layer with photo-crystal layer (https://www.nature.com/articles/s41467-018-03032-2). The interactive mechanochromic smart actuator might also serve as an invisibility cloak imagined in Harry Potter and the Prisoner of Azkaban with the external moisture regulation，however, it is difficult to realize the accurate control of actuation and color change. Compared with the stimuli described above, electricity is an easily and efficiently controllable input factor. Among many electro-responsive materials, there is some scope to realize both electrochromic (EC) and electrochemical actuating functions based on a single device because of their similar reaction and operation conditions. However, the composite films reported before were restricted by the mutual influence of the colors between two functional materials, poor interfacial stability as well as small deformation of nanoporous gold films. Therefore, although the electrical stimulation has shown high efficiency and controllability, a strategy to combine high-performance electrochromism with electrochemical actuation via a unique material or structure becomes the key for this biomimetic technology.

Then, we turned to the electro-control to realize the synchronous transformation in color and shape using single active material. After an arduous experience, it is very exciting for us to achieve this great success. Here, a bio-inspired flexible EC/actuating dual-responsive film was prepared via the construction of AgNW/W18O49NW bilayer networks. These films demonstrate synchronism in pseudocapacitance-induced high-performance EC/actuation. The EC/actuating active material was confirmed as single component, i.e. W18O49NWs, via control experiments. More significantly, reversible deformation mechanism of W18O49NWs was elucidated as the pseudocapacitive lattice contraction/recovery, which was verified through in-situ synchrotron X-ray diffraction, first principles calculations, numerical simulations, and a series of ex situ structural and elemental characterizations. As an extensive application of pseudocapacitive actuation, an unconventional pseudocapacitive IPMC actuator was constructed based on W18O49NWs and demonstrated an obvious displacement. Therefore, this unconventional pseudocapacitive actuating mechanism not only contributes to the technologically relevant electricity-driven color/shape dual-response phenomenon, but also offers the basis for the development of new multifunctional actuators.

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